Linear low-density polyethylene (LLDPE), that is a copolymer of ethylene and .alpha.-olefin, shows higher impact strength when molded into a film, as compared with conventional low-density polyethylene (LDPE) obtained by the high pressure process, so that the linear low-density polyethylene has been broadly used as a film-forming material.
A film obtained from an ethylene/butene-1 copolymer (i.e., an example of the linear low-density polyethylenes) is excellent in tear properties because of its adequate tear strength, but is somewhat low in the impact strength.
A film obtained from a copolymer of ethylene and .alpha.-olefin having 6 or more carbon atoms (i.e, other example of the linear low-density polyethylene) is excellent in the impact strength, but has such a problem that the film cannot be easily torn because of too high tear strength. In other words, the film obtained from such copolymer is bad in the tear properties.
Accordingly, eagerly desired are ethylene/.alpha.-olefin copolymers which can provide films of high impact strength and excellent tear properties. For the purpose of more improving the impact strength of films, a method of using .alpha.-olefin having larger number of carbon atoms has been tried. However, the method sometimes causes such problems that the blocking resistance of the resulting film is much lowered and the transparency of the film is markedly reduced when the film is subjected a heat treatment. Especially in the case of a lamination film, after the film-forming procedure, the obtained film is generally subjected to an aging procedure at approx. 40.degree. C. to enhance the dimensional stability, but in this aging procedure, the film sometimes turns white so that the transparency is reduced.
The present inventors have earnestly studied to solve the above-mentioned problems accompanied by the ethylene/.alpha.-olefin copolymer films. As a result, they have found that if an ethylene/pentene-1 copolymer having been obtained by copolymerizing ethylene and pentene-1 under specific conditions and fulfilling specific requisites is molded into a film, the resulting film is excellent in blocking resistance as well as in a balance between impact strength and tear properties and is remarkably prevented from lowering of transparency caused by a heat treatment. Thus, ethylene/pentene-1 copolymers and processes for the preparation of the same according to the invention have been accomplished.
The present inventors have also found that an ethylene/pentene-1 copolymer composition obtained by adding a specific stabilizer to the above-mentioned ethylene/pentene-1 copolymer is excellent in heat stability in the molding stage, long-term heat stability and weatherability, and that a molded product obtained from the copolymer composition can keep high impact strength and good tear properties both inherently belonging to the ethylene/pentene-1 copolymer. Thus, ethylene/pentene-1 copolymer compositions according to the invention have been accomplished.